1. Field of the Invention
The present invention generally relates to optical phase conjunction, and more specifically to the use of a pseudo-conjugator to produce a retroreflected seed beam in a self-pumped phase conjugate mirror configuration.
2. Description of the Related Art
A phase conjugate mirror produces a wavefront-reversed, or time-reversed reflection of an incident beam, with the phase of the reflected beam reversed from that of the incident beam at all points in space. Several methods of producing phase conjugated beams are known in the art, including four-wave mixing, stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), and stimulated photorefractive scattering (SPS). A detailed treatise on these metods is presented in a textbook entitled "Principles of Phase Conjugation", by B. Ya. Zel'dovich et al, Springer-Verlag, Berlin (1985).
Phase conjugative mirrors can be provided either with external pumping beams, as in a four-wave mixer, or as self-pumped devices. The above referenced related patent application to Dunning utilizes a pseudo-conjugator employed in an externally pumped stimulated photorefractive scattering configuration. The self-pumped configuration is advantageous in that no optical beams other than the input beam are required. In a self-pumped phase conjugator, an input beam is directed into an optical non-linear medium capable of two-wave mixing gain. Scattering of the input beam in the medium results in the creation of a backscattered conjugate beam propagating in the direction opposite to the input beam, which has its phase fronts reversed throughout space relative to the input beam.
Once created, the conjugate beam is capable of being amplified through two-wave mixing with the input beam. In order to create the conjugate beam, a noise threshold condition has to be surpassed. Examples of noise sources in non-linear media include thermal noise and scattering from crystal imperfections. In order for the conjugate beam to be produced and sustained, it must be amplified sufficiently to overcome the noise threshold. In an SPS application, using a material such as crystalline barium titanate, and relying only on unseeded backscattering to produce the conjugate beam, the two-wave mixing gain must be enormous (on the order of e.sup.30), to overcome the noise threshold. For Brillouin and Raman media, the exponent is the product of the material's gain coefficient g with the intensity of the input beam and the interaction length of the non-linear medium. In photorefractive media, the exponent is the product of the photorefractive gain coefficient times the interaction length. The photorefrative gain coefficient is linearly proportional to intensity only at very low intensities; at higher intensities above the equivalent dark intensity, the photorefractive gain coefficient is independent of it.
Sufficient gain to overcome the threshold without any kind of seeding is not always attainable in practical applications, since the input beam intensity, the material's gain coefficient, and/or the length of the medium may not be sufficiently large. Previous approaches to relax this constraint in SPS applications involve the use of "seeding " techniques, which cause the input beam to be reflected back through the non-linear medium as a "seed beam", and which has an intensity larger than the threshold value. Two classes of seeds which have been attempted involve conjugate seeds (a conjugator is placed downstream of the SPS crystal), and "noise seeds", in which case a diffuse reflector is placed beyond the SPS crystal or paiinted on the crystal directly (using, for example, white typewriter correction fluid). In the latter case, a noise source (as opposed to a specularly reflected seed), is required in order to avoid "image print-through", which occurs because the SPS crystal can amplify the reflected seed beam. An example of providing a diffuse conjugate seed is found in an article entitles "SELF-PULSATION AND OPTICAL CHAOS IN SELF-PUMPED PHOTOREFRACTIVE BaTiO.sub.3 ", by P. Gunter et al, Opt. Comm., vol, 55, no. 3, pp. 210-214.
The disadvantages of these two classes of seeds are as follows:
(1) a conjugate seed (even though spatially ideal) requires the use of another phase conjugate mirror, which complicates and adds to the cost of the system considerably; and
(2) a diffuse seed (even though ideal in terms of generating a multiplicity of spatial modes) is inefficient due to the large degree of wide-angle light scattering which results in only a relatively small fraction of the diffusely scattered light finding its way back into the crystal. In addition, the painted diffuse scatterer has, in some cases, led to instabilities in the conjugate reflectivity as described by Gunter et al., presumably due, in part, to a background specular component.
A further threshold related limitation which has existed in the prior art is that only a few crystalline materials, such as barium titanate and in one case lithium niobate, have sufficiently high internal gain and size to be usable in unseeded SPS conjugation applications.
Self-pumped phase conjugate mirrors employing SBS and SRS are generally employed in connection with high power pulsed laser beams, such as from a Nd:YAG laser, but do not work (at least in free-space bulk configurations) with lower power lasers such as the HeNe or argon-ion laser. In addition, the Stokes frequency shift inherent in these systems is undesirable in certain applications.
Previous schemes involving photorefractive interactions inlude external loop geometries as discussed in "Theory and Applications of Four-Wave Mixing in Photorefractive Media", by M. Cronin-Golomb et al, IEEE JQE, QE-20, no. 1 (1984), pp. 12-30, and internal loop geometries as discussed in "Self-pumped, continuous-wave phase conjugator using internal reflection", by J. Feinberg, Opt. Lett. vol 7, no. 10 (1982), pp. 486-488.
Another arrangement is disclosed in U.S. Pat. No. 4,794,605, issued Dec. 27, 1988, entitled "METHOD AND APPARATUS FOR CONTROL OF PHASE CONJUGATION CELLS", to R. Aprahamian et al, which teaches a technique in which one or more phase conjugation cells, such as SBS cells, are controlled by the use of a seed beam injected into the cells. The seed beam is injected at the same frequency and the same direction as the expected phase conjugated beam, and is adjusted to control the phase and other characteristics of the phase conjugated beam. In multiple cell arrays, seed beams are employed to ensure phase coherency of multiple beams. For lower energy applications, use of the seed beam allows a phase conjugation cell to be operated with incident beams of lower energy than would be needed without the seed beam.
Other schemes utilizing the internal geometries of SPS crystals are discussed in "Optical phase conjugation by backscattering in barium titanate", by T. Y. Chang et al, Opt. Lett. vol. 10, no. 8 (1985), pp. 408-410, and "Physics of speckle field interactions in photorefractive crystals", by A. V. Mamaev et al, 1989 CLEO Conference, Baltimore, Md.; paper MD4.
Another method of overcoming the gain limitations in SPS is disclosed in U.S. Pat. No. 4,773,739, issued Sept. 27, 1988, entitled "SELF-PUMPED PHASE CONJUGATE MIRROR AND METHOD USING AC-FIELD ENHANCED PHOTOREFRACTIVE EFFECT", to G. Valley et al, which teaches how to apply an alternating electric field across a photorefractive crystal to establish a photorefractive grating shift of about 90.degree., and bring the crystal gain up to a level at which phase conjugation can take place.
There are at least three important advantages of backseeded SPS over the internal loop-type conjugator. First, since the interaction region in the seeded SPS geometry s distributed over a long length rather than over several very localized interaction regions, it should be easier to obtain accurate coupling of several beams from independent amplifier legs in master oscillator power amplifier applications. Second, since there are not internal loops, along with associated internal corner reflections of the intense filimentary beam, higher damage thresholds should be expected from the seeded SPS geometry. Finally, since the seeded SPS geometry involves the very small period gratings created by counterpropagating beams in the crystal, faster response times can be expected. Application of an external alternating electric field as taught in the patent to Valley may not be practical in certain applications.